UV curing is characterized by a high reaction rate and is frequently used for the coatings of different substrates such as e.g. wood, metal or glass. Thus for example in EP 1 247 843 A2 a UV-curing coating material is described, in which type I photoinitiators such as diethoxyphenylacetophenone or acyl- or bisacylphosphine oxide are used.
WO 01/51533 A1 describes a UV-curing wood-coating material in which acylphosphine oxides, α-hydroxyalkylphenones or α-dialkoxyacetophenones are also used as photoinitiators. Above all, transparent coatings with low layer thickness can be produced with UV curing due to the low wavelength of the UV light. The limits of UV curing are reached with pronounced shading or pigmentation and greater layer thicknesses. Such photopolyreactive resins with clearly reduced transparency cure only incompletely with UV light.
If greater through-curing depths are required, such as for example in the curing of light-curing dental filling materials, visible light is used for irradiation. The photoinitiator system most frequently used for this is a combination of an α-diketone with an amine coinitiator as described e.g. in GB 1 408 265.
Dental compositions in which this photoinitiator system is used are disclosed e.g. in U.S. Pat. Nos. 4,457,818 or 4,525,256, wherein preferably camphorquinone is used as α-diketone. Camphorquinone has an absorption maximum at a wavelength of 468 nm. As a result camphorquinone displays a strong yellow colouring with the disadvantage that materials initiated with camphorquinone/amine often have a yellow cast after curing, as the initiator system is not completely bleached. (N. Moszner, R. Liska, Photoinitiators for direct adhesive restorative materials, In: Basics and Applications of Photopolymerization Reactions, Vol. 1; Fouassier, J.-P., Allonas, X., Eds., Research Signpost, Kerala, 2010, 93-114). This bleaching behaviour is very disadvantageous in particular in the case of bright white shades of the fully polymerized material. In addition, when used in acid adhesives, camphorquinone amine systems have the disadvantage that the radical-forming amine component protonates and is thereby partially deactivated for radical formation.
The use of germanium compounds as photoinitiators is known. Bisacyldialkyl germanium compounds are above all efficient Norrish Type I photoinitiators for curing in the blue light range (B. Ganster, U. K. Fischer, N. Moszner, R. Liska, New photocleavable structures, Diacylgerman-based photoinitiators for visible light curing, Macromolecules 41 (2008) 2394-2400; N, Moszner, U. K. Fischer, B, Ganster, R. Liska, V, Rheinberger, Benzoyl germanium derivatives as novel visible light photoinitiators for dental materials Dent. Mater. 24 (2008) 901-907; N. Moszner, F. Zeuner, I. Lamparth, U. K. Fischer, Benzoylgermanium derivatives as novel visible-light photoinitiators for dental composites, Macromol. Mater. Eng. 294 (2009) 877-886).
EP 1 905 413 A1 and EP 1 905 415 A1 disclose mono-, bis- and triacyl germanium compounds which are suitable as photoinitiators for curing dental materials with visible light. Their synthesis is costly and is carried out starting from expensive dialkyl germanium dihalides using the dithiane protective-group technique and purification using column chromatography.
From EP 2 103 297 A1 suitable acyl germanium compounds which contain several germanium atoms are known as photoinitiators.
WO 2015/067815 A1 discloses bis (germyl)ketones with the formula R1R2R3Ge(CO)GeR4R5R6 and processes for the preparation thereof. These bis(germyl)ketones are also intended to be suitable as photoinitiators for dental materials.